The property returns/sets the RGB String in the form "#RRGGBB".
When the object is created, the value of this property is copied from the PmgTrendViewer.GraphInit object.
If the tvTrend object is connected to the trends server (e.g. by means of calling the Connect method), then it can be set the property to the value of "$default". In this case the value of the property is read from the PmaTrendGroup object from the "Color" configurator.
var oTView = pMe.Items("../TrendView");
var oGraphDef = oTView.Trends("t1").GraphDefault;
oGraphDef.Color = "#00ff00"; //writing